1. Field of the Invention
This invention relates generally to the construction of dental appliances such as crowns and the like. Most specifically, it relates to materials useful as opaque coatings for the metal castings commonly employed in metal-ceramic dental restorations. 2. The Problem and Description of the Related Art
A long-standing aesthetic problem for the wearers of metal-ceramic appliances has been "shadowing" at the gingival margin. This phenomenon is usually manifest as a dark line at the limit of the appliance, such limit commonly being at the same level as the gingival margin. This shadowing is caused by the fact that the opaques heretofore used in the fabrication of such appliances have a tendency to shrink and their edges to become rounded during the glaze firing of the piece. This makes it difficult to realize an accurate marginal finish and additionally allows the metal surface of the base casting to show through the porcelain at the margin, thereby effecting shadowing.
The conventional fabrication technique for a metal-ceramic dental restoration such as a crown begins with the preparation of the supporting tooth by the dentist. First, a shoulder is formed on the tooth to be restored. This shoulder will ultimately support the final installed appliance. A mold is then made of the prepared tooth and from the mold a die is fashioned which is then used to construct the restoration. Using the die, a metal casting is made which conforms exactly to the prepared supporting tooth.
Ultimately, the crown or other such piece is built-up on the metal casting using various dentine and enamel porcelains. However, because these porcelains are translucent, it is first necessary to apply an opaque to the metal casting to prevent its being visible through the outer porcelain layers. Various opaque porcelains for this purpose are well-known in the art. They are usually supplied as a powder which is mixed with distilled water so as to form a suspension or medium which can be applied to the metal casting with a brush. A number of "modelling liquids" are available for use in place of the distilled water to provide a suspension. These are commonly a mixture of distilled water and glycerin. Some include alcohol as a third component.
A principal disadvantage of this fabrication technique is shadowing at the margin or base edge of the restoration. This result obtains because the opaques of the prior art shrink during firing. As they shrink, they recede from the margin of the metal casting allowing the base metal to show through.
John W. McLean, in "The Science and Art of Dental Ceramics----Volume 2," Quintessence Publishing Co., 1980, at page 322 advocates three different techniques for preventing the shadowing caused by the metal casting in metal-ceramic appliances. They are: the platinum foil technique; direct porcelain firing using a refractory ceramic die; and the direct lift-off of porcelain from the shoulders. For the direct porcelain lift-off technique he suggests the use of a dental porcelain mixture comprising one third by weight of core porcelain and two thirds by weight of standard opaque porcelain. Although this porcelain mixture is sufficiently stable to preserve intact the finely modelled sharp edges of the device during firing, problems arise from the different coefficients of thermal expansion possessed by the two components.
Heinz Claus, in a publication entitled "The Development and Application of a Shoulder Porcelain for VMK.RTM.Metal-Ceramics," Dental Magazin, Sept. 1984, Dental Magazin Verlagsgesellschaft mbH, West Germany, describes a material and a technique for constructing a porcelain margin on a dental appliance. The proprietary "shoulder porcelain" described is said to have a melting point approximately 120.degree. C. higher than that of normal dentine porcelain and to have markedly better stability during firing.
The fabrication technique preferred by Claus includes a shortening of the metal casting which is then underlined with paint-on opaque and shoulder porcelain. This technique is said to allow light incident on the tooth to be passed into the prepared tooth underlying the appliance, thereby effectively eliminating the shadowing that would otherwise be caused by the completely light-impervious metal casting.
Shrinkage of shoulder porcelains, however, remains a problem. Thus, Claus states: "To then correct the remaining very slight gap at the margin that still results, despite the markedly reduced shrinkage of these Shoulder Porcelains, the application and firing that have already been described have again to be repeated. . . . Following this correction, the crown should be built-up in the usual way using [conventional]dentine and enamel porcelains, and then fired according to standard firing instructions. One last fine correction to the shoulder should be carried out together with the glaze firing."